The present invention relates to a semiconductor device, such as a power module. More specifically, the invention relates to a semiconductor device which includes a circuit board having an insulating substrate, a first metal plate fixed on a first side of the insulating substrate, and a second metal plate fixed on a second side of the insulating substrate. The semiconductor device further includes a semiconductor element mounted on the first metal plate, and a heat cooling element fixed on the second metal plate.
In a conventional semiconductor device, thermal stress arises at a bond surface between the semiconductor element and the insulating substrate, due to a difference in the linear expansion coefficients of the semiconductor element and the insulating substrate. This thermal stress increases in proportion to the distance from the center of the bond surface. This thermal stress is likely to cause cracking at the bond surface, and warpage of the insulating substrate and the heat radiator. Such cracking or warpage impairs heat transfer performance.
Japanese Unexamined Patent Publication No. 2004-153075 discloses a power module embodying a proposed solution to the above problem. As shown in FIG. 5A, the power module includes an insulating substrate 101, a heat-generating-element 102 fixed on a first side of the insulating substrate 101, a heat radiator 103 soldered to a second side of the insulating substrate 101, and a heat sink 104 fixed to the heat radiator 103 by screws. The heat radiator 103 has a pair of plate-shaped heat radiator bodies 103a formed of a material having a high thermal conductivity, such as a copper. The heat radiator 103 further has a low-thermal-expansion material 105, such as an Invar alloy, intervening between the heat radiator bodies 103a. 
In Japanese Unexamined Patent Publication No. 2006-294699, a heat radiator 120 is proposed. As shown in FIG. 5B, the heat radiator 120 includes an insulating substrate 121 having a first side on which is placed a heat-generating-element 122, a heat sink 123 fixed to a second side of the insulating substrate 121 through a metal plate 124. A stress relaxation member 125, having a stress-absorbing space, is intervened between the insulating substrate 121 and the heat sink 123. The stress relaxation member 125 is formed of a material having a high thermal conductivity, and is metal-bonded to both the insulating substrate 121 and the heat sink 123. The stress relaxation member 125 has stress relaxation holes 126.
Japanese Unexamined Patent Publication No. 2002-176127 discloses another cooling structure for an electronic component 130, and an electric circuit device. As shown in FIG. 5C, in addition to an electric connection part 131 between the electric component 130 and a board 132, the electric circuit device has a thermally-conductive metal layer 133 between the electric component 130 and the board 132. The metal layer 133 is separated into three or more sections with respect to the center of the metal layer 133. One or more side lines of each separated part are slightly rounded by chemical processing (plate processing). In this case, the use of chemical processing significantly increases the time for manufacturing the electric circuit device.
Presently, there is a need for a semiconductor device which has improved cooling performance, in other words, better thermal conductivity from a semiconductor element to a heat radiator, while minimizing the thermal stress.